Create admin user

[altmannmarcelo]

Description

There are some external tools such as ProxySQL/Grafana, that require fetching the status of Readyset, such as SHOW READYSET STATUS, SHOW CACHES, SHOW PROXIED QUERIES.

Currently we need to expose the application user(the one Readyset is configured to use to connect to upstream). This is not ideal. Some reasons:

• This is a security issue - We need to expose the credentials of a user that can potentially change data in the underlining database into multiple systems.
• Operation nightmare - Most organizations rotate their password in a certain cadence. Changing the password in ProxySQL scheduler, Grafana dashboards is error prone.

Admin user should be able to run all the commands from https://readyset.io/docs/reference/command-reference

Change in user-visible behavior

Requires documentation change